by Chris Woodford. Last updated: April 11, 2022.
Want to cook a dinner in five minutes or make an airplane safer to
fly in bad weather? You'll be needing some microwaves, then. Those are the invisible,
super-energetic, short-wavelength radio waves that travel at the
speed of light, doing the important stuff in microwave ovens and
Making microwaves is easy if you have the right
equipment—a handy gadget called a magnetron. What is it and
how does it work? Let's take a
Photo: A wartime (1940s) Sylvania 2J32 cavity magnetron. Photo by John F. Williams courtesy of the Office of Naval Research published on Wikimedia Commons
under a Creative Commons (CC BY 2.0) licence.
How does a magnetron work?
Magnetrons are horribly complicated. No, really—they're horribly
complicated! To understand how they work, I find it helps to compare
them to two other things that work in similar ways: an old-style TV
set and a flute.
Photo: There's a magnetron tucked inside your microwave oven, usually just behind the control and instrument panel on the right. If you open the door, you can sometimes get a glimpse of the magnetron and its cooling fins through the perforated metal cage that separates it from the main cooking compartment.
A magnetron has quite a lot in common with a cathode-ray
(electron) tube, the sealed glass bulb that makes the picture in an
old-style television set. The tube is the heart of a TV: it makes the
picture you can see by firing beams of electrons at a screen covered
in chemicals called phosphors so they glow and give off dots
of light. You can read all about that in our main article on
television, but here (briefly) is what's happening. Inside the TV,
there's a negatively charged electrical terminal called a cathode
that's heated to a high temperature so electrons "boil" off it.
They accelerate down the glass tube, attracted by a positively
charged terminal or anode and reach such high speeds that they
race past and crash into the phosphor screen at the tube's end. But a
magnetron doesn't have the same purpose in life as a TV. Instead of making a
picture, it's designed to generate microwaves—and it does that a little bit
like a flute. A flute is an open pipe filled with air. Blow across
the top in just the right way and you make it vibrate at a specific
musical pitch (called its resonant frequency), generating a
sound you can hear that corresponds directly to the length of the
Artwork: Right: One of the drawings of the high-energy magnetron developed in the 1940s by Percy Spencer, who went on to perfect the microwave oven while working at Raytheon. (I've colored it in to match my own artwork below.) You can see a bigger version of this drawing and read the full technical details via Google Patents. Artwork courtesy of US Patent and Trademark Office.
A magnetron's job is to generate fairly short radio waves.
If you could see them, you could easily measure them with a school ruler.
They're usually no shorter than about 1mm (0.04 in; the shortest
division on a metric ruler) and no longer than about 30cm (12in; the
length of a typical school ruler). The magnetron does its stuff by
resonating like a flute when you pump electrical energy into it. But, unlike a
flute, it produces electromagnetic waves instead of sound waves so
you can't hear the resonant energy its making.
(You can't see that energy either, because your eyes aren't sensitive to short-wavelength, microwave
Photo: Typical microwaves range from the smallest (1mm) division on a school ruler up to the length of the ruler itself (30cm).
A brief history of magnetrons
Photo: The CV64 cavity magnetron, developed in Birmingham in 1942, was small enough to fit inside an airplane. Devices like this made it possible for planes to use radar defenses for the first time. An exhibit at Think Tank (the science museum in Birmingham, England). Sorry about the slightly poor quality of the image: the exhibit is inside a glass case and hard to photograph.
Find out more
On this website
History and development of magnetrons
- Andrei Haeff and the Amazing Microwave Amplifier by Jack Copeland and Andre A. Haeff. IEEE Spectrum, August 25, 2015. Exploring the work of a forgotten figure from microwave history.
- [PDF] The invention of the cavity magnetron and its introduction into Canada and the USA by Paul A. Redhead. Physics in Canada, Nov/Dec 2001. This is an excellent, concise account of how magnetrons developed around the time of World War II in the United States, the UK, and Canada. [Archived via The Wayback Machine.]
- The Cavity Magnetron in World War II: Was the Secrecy Justified? by Bernard Lovell, Notes and Records of the Royal Society of London, Vol. 58, No. 3 (Sep., 2004), pp. 283–294.
- Personalities in Science: Albert W. Hull, Scientific American, Vol. 168, No. 5, May 1943, p. 195. A short biography of the magnetron pioneer—and why his work mattered so much in wartime.
- The Cavity Magnetron: Not Just a British Invention by Yves Blanchard et al, IEEE Antennas and Propagation Magazine, October 2013
- Review of the relativistic magnetron by Dmitrii Andreev, Artem Kuskov, and Edl Schamiloglu. Matter and Radiation at Extremes 4, 067201 (2019). Includes a great review of general magnetron history and many useful citations.
- Historical Notes on the Cavity Magnetron by H.A.H. Boot and J.T. Randall. Transactions of the Institute of Electrical and Electronics Engineers, Number 7, July 1976, p.724. How two British pioneers developed early military magnetrons.
Artwork: Illustrations of Arthur Samuel's original cavity magnetron from his
US Patent #2,063,342: Electron discharge device,
courtesy of US Patent and Trademark Office. As in the artworks above, the anode is colored red, the cathode yellow, and
the coil surrounding the glass discharge tube is dark gray.
If you want to read detailed technical descriptions of how magnetrons are designed and how they work, patents are an excellent place to start. They're not always that easy to understand, but the descriptions are extremely detailed and there are generally very clear labeled diagrams. Here are a few to start you off; you'll find lots more if you search at the USPTO (or Google Patents) using the keyword "magnetron":
- US Patent #2,099,533: Magnetron by Dietrich Prinz, Telefunken Gesellschaft, 30 July 1935. An early German magnetron design.
- US Patent #2,063,342: Electron discharge device by Arthur L. Samuel, Bell Telephone Laboratories, 8 December 1936. The first cavity magnetron.
- US Patent #2,408,235: High-efficiency Magnetron by Percy L. Spencer, Raytheon Manufacturing Company, 24 September 1946. The full text of Percy Spencer's cavity magnetron patent, illustrated up above.
- US Patent #7,906,912: Magnetron by Takeshi Ishii et al Panasonic Corporation, 15 March 2011. A very detailed description of the kind of magnetron you'll find in a modern microwave oven.
- ↑ Personalities in Science: Albert W. Hull.
- ↑ US Patent #2,063,342: Electron discharge device by Arthur L. Samuel.
- ↑ The Cavity Magnetron in World War II: Was the Secrecy Justified? by Bernard Lovell.
Nikolay Alekseyev and Dmitry Malyarov—Lifelines of inventors of multiresonator magnetron by N. A. Borisova, 2011 21st International Crimean Conference "Microwave & Telecommunication Technology", Sevastopol, 2011, pp. 97–99.
- ↑ Historical Notes on the Cavity Magnetron by H.A.H. Boot and J.T. Randall.
- ↑ Review of the relativistic magnetron by Dmitrii Andreev, Artem Kuskov, and Edl Schamiloglu.
Please do NOT copy our articles onto blogs and other websites
Articles from this website are registered at the US Copyright Office. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties.